For instance, in a mobile communication system conducting radio communication in Time-Division Multiplex, such as PHS (Personal Handyphone system), a large number of base stations are placed at appropriate positions within a predetermined area, and thus continuous services are secured by switching base station in radio communication even when communication terminal calling via base station in 1 moves out of service area of such base station.
In the conventional mobile communication system of this type, if there is a difference in frame timing of radio communication between adjacent base stations, service failure occurs and smooth changeover of base station in communication involved in a position shift. Therefore, it is important to synchronize frame timing of each base station.
One of the conventional technology of synchronizing frame timing of each base station is a method to supply synchronizing signals from exchange device (henceforth, PBX), which is a host device of base station (henceforth, BS), to each BS. To explain this conventional technology referring to FIG. 1, synchronizing signal generator unit 107 is provided in PBX 101, and it comprises so as to synchronize each BS by supplying synchronizing signal with same timing to each of BS111 to 113 via BS I/F unit 108 to 110.
Further, a mobile communication system is disclosed in Japanese Patent Application Laid-Open No. 11-68649, which is construed to cut the cost and increase the stability of the system by connecting speech path switch for switching the internal and external lines and by comprising so that each BS can receive synchronizing signal via this speech path.
However, there is a following disadvantage in the conventional synchronization control between base stations.
A communication system of this type has a large number of BS placed within a predetermined area as above, and the distance from the host device PBX to each BS differs according to the position the BS is placed. In this case, arrival timing of synchronizing signal at a BS distant from PBX, i.e. a BS with a long signal transmission path, may be delayed, and thus a difference of synchronizing signals between BS's is produced.
Following are explanations of synchronizing signal delay and a difference of radio transmission timing referring to FIG. 2.
As shown in FIG. 2, three BS 402 to 404 are connected with PBX401, with BS 402 and BS 403 installed at approximately equal distance from PBX 401 and with BS 404 installed in a relatively remote position. From PBX 401, synchronizing signal 408 is output toward each BS with the same timing. When transmission path of BS 404 is longer than the other BS's as this, the timing synchronizing signal arrives at BS404 is later than the other BS's. (See Code 409) Since each BS conducts radio communication with timing of synchronizing signal supplied to it, as for radio communication timing 405 to 407 of each BS, radio communication timing 407 or BS 404 is delayed by Td compared with timing of other BS's.
In such case, for instance, when communication terminal BS is in the position that allows receiving electric wave from BS 403 and 404, n communication failure may occur due to interference of time frame. Further, for instance, when a mobile communication terminal in communication with BS403 shifts its position and switches communication to B404, smooth changeover may be inhibited and fault such as circuit cut off may arise.